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Development status and trend of super duplex stainless steel

What is duplex stainless steel?

Duplex stainless steel (DSS) is a duplex structure in which ferrite and austenite account for about half of the solid solution structure. Because of its unique structural characteristics, through the correct control of chemical composition and heat treatment process, the excellent toughness of austenitic stainless steel is combined with the high strength and chloride stress corrosion resistance of ferritic stainless steel. As a result, duplex stainless steel has the advantages of two phases and has been widely used all over the world. At present, the output of duplex stainless steel in the world is 150000-400000 tons, accounting for only 1% – 3% of the total output of stainless steel, but the average annual growth rate in recent 10 years is rapid It has been widely used in oil and gas, chemical industry, fresh water purification, paper and pulp, food and light industry, as well as construction, building and other structural parts.
At present, the development of duplex stainless steel has gone through three generations: the first generation of DSS is represented by 3re60 steel developed in Sweden in the mid-1960s, which can be used as a chloride ion stress corrosion resistant environment, but has the problem of welding heat affected zone; the second generation DSS is after the 1970s, combined with AOD and VOD The development of refining technology has the characteristics of ultra-low carbon and nitrogen. Its representative brand is SAF2205 developed by Sweden, and the pitting equivalent index (pre) is 32-39; the third generation DSS is super duplex stainless steel (SDSS) with ultra-low carbon, high molybdenum and high nitrogen content developed in the late 1980s )Its representative brands are SAF2507, ur52n +, zeron100, etc., and the pre value is greater than 40.
Since the development of the third generation of super duplex stainless steels, their applications in oil and gas fields, petrochemical and chemical treatment industries have increased rapidly due to their excellent mechanical properties and high corrosion resistance, as well as cost advantages compared with super austenitic stainless steels and nickel based alloy materials with similar properties. However, the corrosion resistance and mechanical properties of duplex stainless steel can not be fully satisfied in the fields with more severe corrosion environment, such as the working conditions of long-term service in hot seawater, and the marine and deep-sea areas that have higher requirements on the comprehensive performance of corrosion resistance / mechanical properties, such as wellhead control system with improved oil and gas recovery methods, and deep-sea submarine pipelines and pipeline cables Meet the use requirements. In addition, from the perspective of cost control, the end user can reduce the cost by thinning the design thickness, which puts forward the demand for higher strength materials.
Therefore, with the increasing requirements and the development of duplex stainless steel in recent years, super duplex stainless steel, a new generation of duplex stainless steel with higher strength and better corrosion resistance, emerges as the times require and gets development and application. This paper gives a brief introduction to UNS S32707 and UNS S33207 in the world In order to represent the development status and trend of ultra super duplex stainless steel, the composition design ideas and organizational characteristics of ultra super duplex stainless steel were analyzed. Combined with its performance advantages, the application examples in related fields abroad were introduced, so as to provide ideas and help for the development of duplex stainless steel in China.

Properties of duplex stainless steel

Due to the characteristics of the two-phase structure, by properly controlling the chemical composition and heat treatment process, the duplex stainless steel has the advantages of both ferritic stainless steel and austenitic stainless steel. It combines the excellent toughness and weldability of austenitic stainless steel with the high strength and chloride stress corrosion resistance of ferritic stainless steel. It is these superior properties that make the duplex stainless steel As a weldable structural material, stainless steel has developed rapidly. Since 1980s, stainless steel has become a kind of steel in parallel with martensitic, austenitic and ferritic stainless steels. Duplex stainless steel has the following properties:

  • (1) Molybdenum containing duplex stainless steel has good chloride stress corrosion resistance under low stress. However, the corrosion resistance of stainless steel in the medium above 60 °C is easy to occur in stainless steel such as stainless steel in medium above 60 °C.
  • (2) Molybdenum containing duplex stainless steel has good pitting corrosion resistance. The critical pitting potential of duplex stainless steel is similar to that of austenitic stainless steel at the same pitting resistance equivalent value (pre = CR% + 3.3mo% + 16N%). The pitting corrosion resistance of duplex stainless steel and austenitic stainless steel is equivalent to AISI 316L. The pitting and crevice corrosion resistance of high chromium duplex stainless steel containing 25% Cr, especially nitrogen, is better than AISI 316L.
  • (3) It has good corrosion fatigue and wear corrosion resistance. Under the condition of some corrosive medium, it is suitable for making power equipment such as pumps and valves.
  • (4) The comprehensive mechanical properties are good. The yield strength is twice as high as that of 18-8 austenitic stainless steel. The elongation of solid solution is 25%, and the toughness value AK (V-notch) is above 100J.
  • (5) It can be welded with 18-8 austenitic stainless steel or carbon steel.
  • (6) The hot working temperature range of duplex stainless steel containing low chromium (18% Cr) is wider than that of 18-8 austenitic stainless steel, and its resistance is small. It can be directly rolled to produce steel plate without forging. The hot working of duplex stainless steel with high chromium (25% Cr) is slightly more difficult than that of austenitic stainless steel, and can produce products such as plate, pipe and wire.
  • (7) The work hardening effect of cold working is greater than that of 18-8 austenitic stainless steel. In the initial stage of deformation, the tube and plate need to be subjected to large stress to deform.
  • (8) Compared with austenitic stainless steel, it has higher thermal conductivity and smaller linear expansion coefficient, which is suitable for lining equipment and producing clad plate. It is also suitable for making tube core of heat exchanger, and its heat transfer efficiency is higher than that of austenitic stainless steel.
  • (9) The high chromium ferritic stainless steel still has all kinds of brittleness tendency, so it should not be used in the working condition above 300 °C. The lower the chromium content in duplex stainless steel is, the less harmful the brittle phases such as σ.

Structure of duplex stainless steel

Duplex stainless steel has the characteristics of both austenitic stainless steel and ferritic stainless steel because of its austenite + ferrite dual phase structure, and the content of the two phases is basically the same. The yield strength can reach 400MPa ~ 550MPa, which is twice of that of ordinary austenitic stainless steel. Compared with ferritic stainless steel, duplex stainless steel has higher toughness, lower brittle transition temperature, better intergranular corrosion resistance and welding performance; meanwhile, it retains some characteristics of ferrite stainless steel, such as brittleness at 475 ℃, high thermal conductivity, small linear expansion coefficient, superplasticity and magnetism. Compared with austenitic stainless steel, the strength of duplex stainless steel is higher, especially the yield strength is significantly improved, and the pitting corrosion resistance, stress corrosion resistance and corrosion fatigue resistance are also improved significantly.

Chemical composition design and organization characteristics

The properties of duplex stainless steel, especially its plastic toughness and corrosion resistance, are closely related to the equilibrium ratio of two phases and the control of harmful phases. The phase composition and phase ratio are mainly determined by the chemical composition and solution treatment process of the steel. The chemical composition design and heat treatment process of super duplex stainless steel also follow this basic principle.

Composition design

In order to meet the increasing application performance requirements, high alloying is the design idea of super duplex stainless steel. However, several principles should be considered in the composition design: stable two-phase equilibrium structure, good thermal stability and as few harmful precipitates as possible. Therefore, it is necessary to increase the content of Cr, Mo and N in the steel to improve the strength and corrosion resistance, and at the same time to adjust the reasonable proportion of other elements to obtain excellent comprehensive properties.
The pre value of super duplex stainless steel with higher strength and better corrosion resistance is close to 50 (PRE= w Cr +3.3 × (w Mo +0.5wW+16wN), can meet the requirements of more severe corrosive medium environment and modern industrial development.

Phase ratio control

When the chemical composition of the steel is constant, the proper austenite / ferrite phase ratio can be controlled by solution treatment at appropriate temperature. Figure 1 lists the phase proportions in the microstructure of UNS S32707 steel after different solution temperatures. It can be seen that there is a certain amount of harmful phase σ phase in the structure when the temperature is lower than 1100 ℃. At higher temperature, with the increase of solution temperature, the proportion of ferrite phase in steel increases gradually, while the proportion of austenite phase decreases accordingly.
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Fig.1 Effect of solution temperature on phases of UNS s32707 steel
Figure.2 shows the microstructure of UNS s32707 after solution treatment at 1100 ℃. The bright part with island distribution is austenite phase, and the darker part is ferrite phase. At this time, the ferrite phase and austenite phase account for about half, and the composition of ultra super duplex stainless steel is in balance.

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Fig.2 Microstrueture of UNS S32707

In addition, in order to prevent the weak phase from being preferentially corroded, the pre values of austenite and ferrite phases of steel are designed to be equivalent. Taking UNS s32707 as an example, the average pre values of two phases are less than 1.

Corrosion resistance

Pitting and crevice corrosion are the most common failure modes in stainless steel in chloride solutions containing oxidants. The corrosion resistance of duplex stainless steel can be improved by adding chromium, nitrogen and molybdenum. This experience has also been fully used in the research and development of super duplex stainless steel. According to the data in Table 1, the theoretical pitting resistance equivalent of UNS S32707 and UNS S33207 is about 50. The critical pitting temperature (CPT) of duplex stainless steel was tested according to astmg48a revised standard. The results showed that the critical pitting temperature of UNS S32707 and UNS S33207 were higher than 90 ℃, while that of UNS S32750 was about 80 ℃.
The increase of critical crevice corrosion temperature (CCT) is also very important for the use of materials, which will greatly improve the service temperature of materials in chloride containing media with low redox potential, such as industrial steam in refineries, seawater, especially in media such as low pH and high concentration chloride solution Corrosion resistance is much better than UNS S32750 and super austenitic stainless steel 254SMO. In industrial production, stress corrosion cracking (SCC) is the most serious form of corrosion, which may lead to rapid failure of materials. Like other duplex stainless steels, UNS S32707 can be used in chloride ion corrosion environment to avoid stress corrosion cracking due to its austenite and ferrite dual phase structure. The data comparison of super duplex stainless steel, super duplex steel, duplex stainless steel and 300 series austenitic stainless steel in 1000 h chloride ion resistance SCC test is shown in Fig.4. It can be seen from Figure.3 that the stress corrosion cracking resistance of ultra super duplex stainless steel is more excellent. Uniform corrosion affects the service life of stainless steel parts in the medium, while UNS S32707 has good corrosion resistance in organic acids such as formic acid and acetic acid. In the environment where standard austenitic stainless steel is corroded rapidly, super duplex stainless steel can even replace high alloy austenitic stainless steel and nickel base alloy.
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Fig.3 SCC resistance of DSS grades and austenitic stainless steels

Mechanical property 

The yield strength of super duplex stainless steel is twice as high as that of austenitic stainless steel with pitting corrosion resistance. Higher strength can significantly reduce material thickness, quality and installation cost.
At the same time, it still keeps good plasticity, and the manufacturing process is similar to that of austenitic steel. The yield strength and tensile strength of UNS S32707 for heat exchanger tubes are generally 800MPa and 1000MPa respectively, and the elongation under quenching heat treatment is more than 25%. The tensile strength of UNS S33207 is 20% higher than that of UNS S32750 under quenching heat treatment.
The temperature range of duplex stainless steel ranges from – 50 ℃ to 300 ℃.
Because of the ferrite phase, the material will be brittle at low temperature; when the decomposition rate of ferrite reaches the maximum at higher temperature, the ferrite phase will transform and become brittle (475 ℃). However, UNS S32707 has good impact toughness and its brittle transition temperature is lower than – 50 ℃.

Application of super duplex stainless steel

Marine / deep sea environment

Ultra super duplex stainless steel with ultra-high strength and corrosion resistance is very suitable for marine environment, especially for deep-sea oilfield exploitation.
As a high alloy dual phase steel, UNS S32707 is especially suitable for harsh, acidic or chloride corrosion environments, such as tropical seawater. Sandvik company has developed saf2707hd wire rod for offshore oil wells under chloride ion corrosion environment. It can be used in the temperature range of – 50 ~ 280 ℃, and can resist high temperature chloride ion point corrosion. After 6 months of testing, saf2707hd material has a long service life, shortens the shutdown period of oil wells, reduces the cost and maintenance costs. It is an ideal material for oil wells, and has been successfully applied to offshore deepwater wells.
The tensile strength and failure load of the material are higher than that of most stainless steel and high alloy wire rod. High strength, wear resistance, mechanical damage resistance and good fatigue strength can effectively prolong the life expectancy of the material. The result of ssun765 is twice as long as that of ssun707 in Indonesia. This material can be used under the conditions of Cl – content of 15000 × 10 – 6, 153 ℃ and 34mpa. After long-term use, the ductility of UNS S32707 wire remains good, and it has become a low-cost alternative material for super austenitic stainless steel and nickel base alloy.
The submarine umbilical is used to connect the offshore drilling control platform and the subsea oil wells. It plays the role of transmitting signals and transporting chemicals to the subsea oil and gas wells. It is a key component of the ocean, especially the deep-sea oil and gas exploitation. The material of submarine umbilical tube not only needs more stringent requirements of strength and corrosion resistance, but also needs to withstand high cycle and low cycle fatigue. Previously, ultra duplex stainless steel UNS S32750 pipe was widely used. However, with the increase of drilling depth, when the water depth of some new projects reaches 2500 m and the water pressure reaches 103 MPa, the super duplex stainless steel material can still be used, but the pipe wall thickness and coating must be increased to meet the design requirements of strength and corrosion resistance. With the increase of pipe wall thickness, the super duplex stainless steel can not even bear the self weight, and the installation cost increases greatly. The application of the new generation of ultra super duplex stainless steel UNS S33207 pipe makes the development of ultra deep subsea oilfield possible. Compared with super dual phase steel, saf3207hd developed by Sandvik company has higher strength (980 ~ 1180mpa) and similar elongation, and has high fatigue cycle life, which can work at about 90 ℃.
Its good mechanical properties make the pipe wall thickness and equipment quality greatly reduced, and the installation cost also reduced. Compared with SAF2507 material, the thickness of umbilical tube wall can be reduced by 21% and the weight can be reduced by 22%, which makes it possible to develop deep-sea oil fields with a depth of more than 2500 m.

Desalination / seawater heat exchanger

As early as the 1970s, 329j1 and ntkr-4 (00cr25ni5mo1.5-2.5-containing or not containing nitrogen) steel began to be used in Japan to manufacture tube bundles of seawater heat exchangers or seawater coolers for chemical plants or ships, partially replacing aluminum brass tubes. The main material used at present is UNS S32750 super duplex stainless steel. Compared with super duplex stainless steel (the maximum service temperature is 50 ℃), the ultra super duplex stainless steel can be used at 70 ℃ with higher operation efficiency. For example, the seawater heat exchanger of a chemical plant in the Mediterranean Sea has butane containing a small amount of fluoride on the shell side, and the inlet temperature is 80 ℃, the outlet temperature is 30 ℃, and the tube side is seawater. In the past, titanium pipes that are not resistant to fluoride ion corrosion were only used for three months. The factory finally selected SAF2707HD super duplex stainless steel from Sandvik company to make seawater cooling exchanger. Its performance is more reliable than SAF2205. It has been used since the end of 2005.
Therefore, in seawater desalination or seawater heat exchanger environment, super duplex stainless steel has great advantages. If the ultra super duplex stainless steel is considered, the maintenance cost will be greatly reduced, the operation time will be prolonged, and the equipment operation will be more efficient and safe. The ultra super duplex steel will have a real low life cycle cost.

Oil and gas industry

Since 1980s, the amount of duplex stainless steel in the production of acid gas and oil has increased gradually, mainly used as production pipe lining, onshore and offshore pipeline system and heat exchanger. Especially in the production of natural gas, most duplex stainless steels are faced with acidic environment containing a lot of Cl -, CO2 and H2S. At the end of 2003, due to the low grade of crude oil used in an American oil refinery, it was easy to cause chloride ion corrosion. Previously, the super duplex stainless steel UNS S32750 was usually used. However, under harsh operating conditions, pitting corrosion and even cracking occurred in the pipe material. However, the pipe material of the top pump and the heat exchanger of the atmospheric distillation unit was fully used So far, UNS S32707 has been running very well.
A European refinery also had a similar problem. The carbon steel used in the air condenser on the top of atmospheric distillation unit had a very low service life. However, the condenser tube made of UNS S32707 material was installed in the worst environment of the air condenser, and the service life was good after one year of service.

Pulp and paper industry

Duplex stainless steel has been used in pulp and paper industry for 40 years. 3re60 steel, UNS S32205, UNS S32750 and other duplex stainless steels have been applied. The advantages of duplex stainless steel, such as excellent mechanical properties, corrosion resistance, corrosion fatigue resistance and stress corrosion resistance, have achieved good results in the production of sawdust pre steamer, intermittent and continuous pulp digester, papermaking pressure drum and recycling equipment for pulp and paper industry. In addition, in developed countries such as North America, Europe and Japan, (super) duplex stainless steel has been successfully used in oil refining industry, chemical fertilizer industry, energy and environmental protection engineering, light industry and food engineering, and high strength structural parts. At present, the use of super duplex stainless steel has become a trend. For example, a chemical plant producing boric acid and borate salt in South America originally used 6Mo super austenitic stainless steel. Due to the high temperature environment with high chloride content and corrosion problem, UNS S32707 pipe was used in the plant to replace it in 2005.

Conclusion

Super duplex stainless steel has been developed and applied in foreign countries for many years. Under the promotion of production enterprises and associations, it is getting more and more users’ favor and using more and more widely.

At present, the mainstream brand of China is still 2205 traditional duplex stainless steel. There is a small amount of production of super duplex stainless steel, while ultra super duplex stainless steel is just starting and in the stage of research and development. Considering the strategic needs of China’s sustainable development, such as deep-sea oil and gas exploitation, seawater desalination, flue gas desulfurization, new energy and other fields are imminent, and these are exactly the places where ultra super duplex stainless steel can play its advantages. Therefore, in order to promote the development of super duplex stainless steel in China, it is necessary to organize the upstream and downstream combination of manufacturers and users and the joint research of production, teaching and Research on the key technologies of composition design, hot working performance and welding performance of ultra super duplex stainless steel in China, and strengthen the technical exchange at home and abroad, increase publicity efforts by using various means, promote the application field, and promote the development of ultra super duplex stainless steel in China.

Source: China Duplex Stainless Flanges Manufacturer – Yaang Pipe Industry (www.epowermetals.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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